1. Field of the Invention
The present invention relates generally to secondary batteries miniaturized for example in forms of buttons, coins and the like and used as a main power source for electronics, a power source for backing up memory and the like, an anode or negative electrode cans thereof, and methods of manufacturing the same, and more specifically to anode cans of secondary batteries obtained by bending a material formed of layers stacked to combine materials different in hardness, secondary batteries using the anode can, and methods of manufacturing the same.
2. Description of the Background Art
FIG. 25 is a schematic partial cross section of a conventional battery. With reference to the figure the conventional battery will be described.
The FIG. 25 battery is a so-called manganese (Mn)-lithium (Li) secondary battery and it includes a cathode or positive electrode can 101 also serving as a cathode terminal, an anode or negative electrode can 102 connected to cathode can 101 via a gasket 106 and also serving as an anode terminal, and a cathode 107, a separator 109 and a lithium metal 108 accommodated in a space formed by cathode and anode cans 101 and 102. Cathode can 101 is formed by shaping a stainless steel plate highly resistant to corrosion. Anode can 102 is formed by shaping a material including a stainless steel layer 102a formed of the same stainless steel that forms cathode can 101, and a hard aluminum alloy layer 102b arranged on an inner circumference of stainless steel layer 102a. Note that the FIG. 25 battery has a structure in symmetry relative to a centerline 111.
Anode can 102 has a periphery provided with a shoulder 103 lower by one step than an upper surface of anode can 102. At the outer portion of the shoulder 103 the periphery is angled 104. From angled portion 104 a peripheral wall 105 extends in a downward direction substantially vertically. Anode can 102 thus includes shoulder 103, angled portion 104 and peripheral wall 105.
Between cathode can 101 and anode can 102 there is arranged cathode 107 on cathode can 101. Cathode 107 is covered by separator 109. On separator 109, lithium metal 108 forming an anode is arranged in contact with hard aluminum alloy layer 102b of anode can 102. Cathode 107, separator 109, lithium metal 108 and an electrolyte configure a power generation cell.
A gasket 106 electrically insulates cathode can 101 and anode can 102 and also closely seals cathode 107, separator 109 and lithium metal 108 in a casing formed by cathode and anode cans 101 and 102. Gasket 106 is arranged between an internal surface of an erected portion 101a of a periphery of cathode can 101 and an outer peripheral surface of anode can 102 extending from shoulder 103 to peripheral wall 105 and erected portion 101a is then folded to seal the battery.
The present inventor studied the FIG. 25 conventional battery and has found that it has disadvantages, as described hereinafter.
More specifically, in the FIG. 25 battery, anode can 102 is pressed to form shoulder 103, angled portion 104, peripheral wall 105 and the like. Anode can 102, however, is formed of a material formed by a stack of layers including stainless steel layer 102a and hard aluminum alloy layer 102b, i.e., a clad material. As such, when the material is pressed, as described above, its peripheral wall having been pressed has an end, as shown in FIG. 26. More specifically, layer 102b, arranged inside layer 102a, has an end 126 protruding relative to an end surface 116 of layer 102a (or layer 102b has an end surface 117 in a region protruding relative to end surface 116 of layer 102a). FIG. 26 is a schematic diagram for illustrating a disadvantage of the FIG. 25 battery. Furthermore, as can be understood from FIG. 26, end 126 is formed as layer 102b being pressed, as described above, is plastically deformed, extruded from an end of the layer. End 126 thus has a geometry having a gently curving surface, as shown in FIG. 26.
Furthermore, in some case, anode can 102 is angled 104 by pressing it at peripheral wall 105 vertically using a die and a punch. This results in peripheral wall 105 having an end such that hard aluminum alloy layer 102b has an end 127 covering an end surface of stainless steel layer 102a, as shown in FIG. 27. FIG. 27 is another schematic diagram for illustrating a disadvantage of the FIG. 25 battery.
When hard aluminum alloy layer 102b has end 126 protruding and having a curving surface, as shown in FIG. 26, or it has end 127 covering an end surface of stainless steel layer 102a, as shown in FIG. 27, there is a case in which anode can 102 cannot have peripheral wall 105 firmly fixed to gasket 106 (or peripheral wall 105 cannot have an end plunged into and fixed in gasket 106) when the battery is sealed. As a result, gasket 106 and anode can 102 bonded together provide poor hermeticity and the battery thus has poor characteristics, a reduced lifetime (or charging and discharging cycle lifetime), and other similar disadvantages.